This invention is most applicable to video display terminals or small computing systems which generate a video display based upon a set of digital graphic characters stored in memory. A typical system of this type would employ a video screen resolution of about 256.times.192 individual picture elements or pixels. Assuming that a one bit code is assigned to each picture element, that is whether the picture element is one (white) or off (black), then approximately 6K bytes of memory would be required for complete storage of one screen. This storage requirement would be substantially increased in the case in which a color video display is desired. Typically, eight or sixteen different colors are available in a color video display thereby multiplying the memory requirement for complete storage of one screen by a factor of three, in the case of a selection of eight different colors, or four for the case of a selection of sixteen different colors. Thus a 256.times.192 picture element system employing a choice of sixteen colors would require approximately 24K bytes of memory.
A screen resolution of 256.times.192 picture elements is typically below the resolution that an ordinary television set or video monitor can display. The screen resolution ordinarily found in a good quality television broadcast system is approximately 512.times.384 picture elements. In order to completely store graphic signals for a sixteen color system of this resolution approximately 96K bytes of memory capacity would be necessary. This amount of memory capacity greatly exceeds the amount of random access memory typically provided in video display terminals or small computing systems of this type.
In order to overcome this requirement for a large amount of memory in order to present graphic characters, video display terminals and small computing systems of this type typically utilize several techniques. Firstly, the screen in these systems is typically provided with only approximately 256.times.192 picture elements of resolution. This picture resolution is provided by quantatizing the minimum length of time for each picture element within each scan line, thereby providing a horizontal resolution of approximately 256 picture elements. In addition, it is not typical to employ an interlace scan in such systems. This is, each video frame has 192 scan lines and each of these scan lines are transmitted sequentially from the top to the bottom of the display. This is different from the television broadcast techniques most often used, particularly this is different from the NTSC standard employed in the United States and Japan and PAL standard most commonly employed in Europe. Under these systems, the video frame is scanned in two half frames. These two half frames include a first half frame in which the scan signals for the odd number scan lines are transmitted and a second half frame in which the scan signals for the even number scan lines are transmitted. In addition, a video display terminal or small computing system of this type typically would employ a finite set of graphic character words to define the picture video attributes of a group of picture elements. Typically one graphic character would define the video attributes of an 8.times.8 group of picture elements. Then the screen would be defined by a listing of the addresses within the memory where the graphic character words for the particular screen are stored.
Even with the use of such memory reduction techniques, typically such a video display terminal or small computing system must make a trade-off between the amount of memory space employed and the video resolution generated. The compromise is most often struck in the favor of limited memory and limited screen resolution. This results in a problem of a jagged "staircase" edger on diagonal lines. This "staircase" type diagonal line typically provides a less attractive video display than if such lines could be smooth.